TABLE OF CONTENTS Table of Contents Executive Summary i vii 1. Introduction 1-1 1.1 Background 1-1 1.2 Purpose and Scope 1-2 1.3 Organization 1-4 2. Land and Water Resources 2-1 2.1 Land Use 2-1 2.2 Topography and Watersheds 2-4 2.3 Soils 2-6 2.4 Key Water Resources 2-7 2.5 Existing Flood Insurances Studies 2-9 2.6 Other Natural, Biologic, and Water Resources 2-10 3. Goals, Policies and Guidelines 3-1 3.1 Purpose 3-1 3.2 Background 3-1 3.2.1 2020 Vision and Strategic Plan 3-1 3.2.2 Comprehensive Plan 2020 3-2 3.3 City of Prior Lake SWMP Goals and Policies 3-3 3.3.1 Water Quantity 3-4 3.3.2 Water Quality 3-6 3.3.3 Recreation and Fish and Wildlife 3-7 3.3.4 Enhancement of Public Participation; Information and Education 3-7 3.3.5 Public Ditches 3-8 3.3.6 Groundwater 3-8 3.3.7 Wetlands 3-8 3.3.8 Erosion and Sediment Control 3-9 3.3.9 Prior Lake NPDES Permit 3-10 3.3.10 Financial Management 3-11 3.4 County, State and Federal Agency Requirements 3-11 City of Prior Lake i
3.4.1 Minnesota Department of Natural Resources 3-11 3.4.2 U.S. Army Corps of Engineers 3-12 3.4.3 Board of Water and Soil Resources 3-12 3.4.4 Minnesota Pollution Control Agency 3-12 3.4.5 Environmental Protection Agency 3-13 3.4.6 Prior Lake Spring Lake Watershed District and Scott County Watershed Management Organization 3-13 3.4.7 State and Federal Jurisdictional Boundaries for Public Wetlands and Waters 3-13 3.5 Agency Contacts 3-16 3.6 Water Resources Management-related Agreements 3-17 3.7 Impacts of the Prior Lake LWSMP on other Units of Government 3-17 3.8 Watershed Goals and Strategies that Affect the City of Prior Lake 3-18 4. Wetland Management Plan 4-1 4.1 Wetland Inventory Goals 4-1 4.2 Wetland Identification 4-1 4.3 Wetland Mapping 4-2 4.4 Wetland Evaluation Methodology 4-2 4.4.1 Minnesota Routine Assessment Method 4-2 4.4.2 Database 4-3 4.5 Required Submittals at the Time of Development 4-3 4.6 Wetland Ranking Methodology 4-3 4.6.1 Habitat Protection/Wetland Ranking 4-3 4.6.2 Stormwater Protection Ranking 4-4 4.7 Wetland Management Standards and Recommendations 4-7 4.7.1 Water Quality 4-7 4.7.2 Water Quantity 4-8 4.7.3 Wetland Buffer Strip and Setback Protection 4-8 4.8 Wetland Restoration/Enhancement Opportunities 4-11 4.9 Wetland Stewardship 4-11 4.9.1 Enhancement 4-12 4.9.2 Control of Invasive Exotic Species 4-12 4.9.3 Habitat Structures 4-12 4.9.4 Learning Opportunities 4-13 5. System Assessment and Design 5-1 5.1 General 5-1 5.2 System Assessment 5-2 5.2.1 Water Quality Assessments 5-2 5.2.1.1 Clean Water Act Assessments 5-2 5.2.1.2 PLSLWD Assessments 5-4 5.2.1.3 Scott County Water Management Organization 5-8 5.2.2 Water Quantity Assessments 5-8 5.2.2.1 City Identified Problem Areas 5-8 5.2.2.2 Pike Lake and Jeffers Pond Districts 5-8 City of Prior Lake ii
5.2.2.3 PLSLWD Volume Management 5-10 5.3 System Design 5-12 5.3.1 Hydrologic Modeling Discussion 5-12 5.3.2 Design Recommendations and Discussion 5-15 5.3.2.1 Conveyance and Storage System Concepts 5-15 5.3.2.2 Water Quality System Concepts 5-20 5.3.2.3 The Use of Wetlands in the Surface Water System 5-24 5.4 System Description 5-27 6. Implementation Plan 6-1 6.1 General 6-1 6.2 Cost Analysis and Capital Improvement Plan 6-1 6.2.1 Cost Estimation Methods 6-2 6.2.1.1 Pipe Costs 6-2 6.2.1.2 Pond Costs 6-3 6.2.2 System Costs and Capital Improvement Plan 6-4 6.3 Financing and Cost Recovery 6-7 6.3.1 Area Charges and Cost Recovery Calculations 6-7 6.3.2 Area Charge Summary 6-9 6.4 NPDES Permit 6-9 6.5 Operation and Maintenance 6-9 6.5.1 Activities 6-9 6.5.2 Stormwater Basins 6-10 6.5.3 Sump Manholes and Sump Catch Basins 6-12 6.5.4 Storm Sewer Inlet Structures 6-12 6.5.5 Open Channels 6-13 6.5.6 Piping System 6-13 6.5.7 De-Icing Practices 6-13 6.5.8 Street Sweeping 6-14 6.5.9 Detection of Illicit Connections 6-14 6.6 Education 6-15 6.6.1 General 6-15 6.6.2 City Staff 6-15 6.6.3 City Residents 6-16 6.6.4 Development Community 6-16 6.7 Financing and the Storm Water Utility 6-17 6.7.1 Current Status Summary 6-17 6.7.2 The Storm Water Utility into the Future 6-17 6.8 Design Standards 6-19 6.9 Watershed Implementation Priorities 6-19 6.10 City of Prior Lake Implementation Priorities 6-20 6.11 Amendment Procedures 6-20 6.11.11 Minor Amendments 6-21 6.12 Annual Report to Council 6-21 City of Prior Lake iii
7. Summary and Recommendations 7-1 7.1 Summary 7-1 7.2 Recommendations 7-2 Appendices Appendix A Appendix B Appendix C Appendix D Appendix E Drainage Areas Trunk Storm Sewer Data Pond Data Stormwater System Costs Wetland Ranking Figures and Maps Figure 1 Location Map 1-6 Figure 2 Land Use Plan 2-2 Figure 3 Annexation Phasing Plan 2-3 Figure 4 Public Waters: Water Basin 3-15 Figure 5 Public Water: Water Course 3-16 Figure 6 Overall Functional Ranking Flow Chart 4-6 Map 1 Surface Water System Back of Report Map 2 Surface Water System Back of Report Map 3 Surface Water System Back of Report Map 4 Surface Water System Back of Report Map 5 Surface Water System Back of Report Map 6 Surface Water System Back of Report City of Prior Lake iv
List of Tables Table 1.1 Population and Households 1-2 Table 2.1 Land Use Based Peak and Volume Comparisons 2-4 Table 2.2 Soil Drainage Characteristics 2-6 Table 2.3 Hydric Soils of Scott County 2-7 Table 2.4 Flood Insurance Study Results 2-10 Table 4.1 Wetland Community Susceptibility to Stormwater Impacts 4-7 Table 4.2 Stormwater Protection Standards 4-7 Table 4.3 Wetland Quantity Standards 4-8 Table 4.4 Recommended Buffer Strip Features 4-10 Table 5.1 Table 5.2 303(d) 2004 Final List of Impaired Waters Within the City Of Prior Lake and its 2020 Growth Area 5-3 Growing Season (May-Sept) Average Lake Monitoring Results, 2000-2003 From PLSLWD 2003 Annual Report 5-5 Table 5.3 Carlson s Trophic State Index (TSI) Explanation 5-6 Table 5.4 Relationship of MCES Lake Grade to Trophic Status 5-6 Table 5.5 Tropic Status of District Lakes, 2003 5-7 Table 5.6 PLSLWD Memo Table #1: 2-YR Peak Flow Rates 5-10 Table 5.7 PLSLWD Memo Table #2: 100-YR Peak Flow Rates 5-10 Table 5.8 PLSLWD Volume Study Results Summary 5-11 Table 5.9 Runoff Coefficients 5-14 Table 5.10 Phosphorus Concentrations and Export Coefficients 5-22 Table 5.11 Benefits of Wet Detention Ponds 5-22 Table 5.12 Wetland Community Susceptibility to Stormwater Impacts 5-25 City of Prior Lake v
Table 5.13 Drainage Districts 5-28 Table 5.14 Comparison of SWMP Modeled Flows to 2003 City of Prior Lake/PLSLWD Agreement 5-35 Table 5.15 Potential Retention Volume 5-40 Table 6.1 Capital Improvement Plan 6-4 Table 6.2 Land Use Based Equivalency Factors 6-7 Table 6.3 Stormwater Area Charge 6-8 Table 6.4 Wet Pond Maintenance Schedule 6-10 Table 6.5 Stormwater Utility Revenue 6-17 Table 6.6 Future Stormwater Utility Funding 6-18 City of Prior Lake vi
EXECUTIVE SUMMARY This report provides the City of Prior Lake with a (LSWMP) that will serve as a policy basis for the management of the surface water system throughout the City. The LSWMP is intended to complement the City s Comprehensive Plan 2030 and official controls governing stormwater. The LSWMP will carry the City through the end of 2015. Periodic amendment to the Plan will occur in the intervening 10 years so that the Plan remains current to watershed plan amendments and revisions and current to the state of the art in surface water management. The Prior Lake LSWMP will serve as a comprehensive planning document to guide the City in conserving, protecting, and managing its surface water resources. The LSWMP meets requirements as established in Minnesota Rules 8410. In addition, the participation of other organizations, particularly Scott County WMO and Prior Lake Spring Lake Watershed District, ensures the City s compliance with local and regional expectations. The City will submit its LSWMP to Metropolitan Council, Scott County WMO, and PLSLWD for review. These entities have 60 days for their review after written receipt of the City Plan. Three other activities complement the LSWMP. The Wetland Management Plan (WMP) provides an assessment and management plan for numerous wetlands within the 2030 growth area. The WMP is based upon standard assessment methodology and is utilized, in conjunction with the LSWMP hydrologic modeling, to determine future use of wetland basins for storage, retention, and infiltration. The WMP constitutes section 4 of this report. The second activity is the Upland Management Plan. Like its wetland counterpart, this Plan provides an assessment and management plan for resources this time upland resources. The City will utilize this information in open space and park planning. The third activity is the Public Works Design Manual (PWDM). The PWDM summarizes the policy and recommendations set forth in the LSWMP and provides design standards and a method of enforcing water resource management concepts detailed in the LSWMP. This report is a culmination of the activities described above and is organized as follows: City of Prior Lake vii
Section 2, Land and Water Resources Inventory, describes the physical environment including watersheds and drainage patterns, dominant land uses, and significant water bodies within the City. Section 3 - Goals, Policies and Guidelines - lists the City s goals and policies along with public agency requirements affecting surface water management in the City. Section 4, Wetland Management Plan, presents the results of an assessment of the City s larger wetlands within the 2020 growth areas. The Wetland Plan identifies specific strategies for mitigating wetland impacts often associated with development. Section 5, System Assessment and Design, presents an overview of all the major watersheds in the City. This section describes in detail the affect rural drainage has on municipal systems both now and in the future. Section 5 also provides detail on a model of the storm water management system within the four focus areas. The focus areas are soon to develop portions of the larger study area where conceptual ponds and trunk pipes are sized and shown, where trunk alignments are shown, and where volumes, discharge rates, and capital costs are analyzed. Section 6, Implementation Plan, covers regulatory responsibilities, priority implementation items, educational programs, operation and maintenance, the capital improvement program, and financing considerations. A plan amendment process is also identified and the distinction between major and minor amendment outlined. Section 7, Summary and Recommendations, contains a summary the SWMP and makes recommendations for implementing the Plan. It should be noted that the land use plan identifies future land use for areas within the 2030 growth boundary. Service areas for the sewer and water system can be effectively defined by this boundary. In contrast, the surface water system is defined by topography and the drainage that currently moves through the newly developing areas must continue to be accommodated in the post development condition. For this reason, the modeling and management strategies incorporated in the LSWMP must deal substantively with the large rural and agricultural areas that will continue to drain through the City even after build out of the 2030 Plan. For instance, drainage to Prior Lake extends as far south as the PLSLWD boundary. This drainage extends almost to Cynthia Lake, three miles south of the growth boundary and incorporates the discharge from Fish Lake, which is over 2.5 miles south of the growth boundary. So, while the 2030 Land Use Plan forms the basis of the urban system outlined in this report, this urban system is also determined by these large rural drainage areas discharging to Prior Lake. The intent of the ponding system described in this report is to reduce the post development peak to a rate more in line with natural conditions. This protects the city s lakes, wetlands, and channels from erosion and flooding. Volume control, though not specifically required by the LSWMP, is aimed at reducing the post development runoff depth and is included as a requirement in the PWDM. By reducing the post development runoff depth through volume reduction and infiltration to something more akin to the depths seen off the natural landscape lake, wetland and channel protection is City of Prior Lake viii
augmented. Infiltration will also help achieve the PLSLWD s retention goals which are outlined later in this report. A change in land use from agricultural and natural to urban is the primary factor driving the need for the Prior Lake Surface Water Management Plan. The goal of the plan is to mitigate the impacts caused by urbanization. Most of Prior Lake falls within the jurisdiction of the Prior Lake Spring Lake Watershed District since, quite obviously, most the City s drainage both current and within the 2030 growth area ends up in either Prior Lake or in the Prior Lake outlet channel. A portion of the City and City 2030 growth area falls within the Scott County WMO. This area lies northwest of Spring Lake and generally drains toward Louisville Swamp, which lies approximately 2 miles west of the 2030 growth area boundary. The Prior Lake Spring Lake Watershed District encompasses approximately 42 square miles of land in the jurisdictions of five local units of government: Prior Lake, Savage, Shakopee, Sand Creek Township, and Spring Lake Township. Most of the district s land area falls within Prior Lake s current limits and 2030 growth area. The primary water resources within the district, which are discussed in detail below, include Spring Lake, Upper and Lower Prior Lakes, Rice Lake, and Crystal Lake. Jeffers Pond and Pike Lake are notable water resources that form a portion of the Prior Lake outlet channel. Historically, three other watershed management organizations operated near the City of Prior Lake. These were the Sand Creek, Credit River, and Shakopee Basin WMOs. All three WMOs were determined to be non-implementing and subsequently they were disbanded by the state Board of Soil and Water Resources. Scott County then assumed the powers of these organizations through creation of the Scott County WMO. The Scott WMO includes all of Scott County not currently managed by the PLSLWD, the Lower Minnesota River Watershed District, the Vermillion WMO or the Black Dog WMO. The primary Scott WMO hydrologic features within Prior Lake's existing or 2030 boundary include Campbell, Markley, Mystic and Howard Lakes. This Goals and Policies section of the SWMP outlines goals and policies specific to surface water management in Prior Lake and its environs. The goals and policies are broad statements regarding the motivation and intent of the SWMP. The policies that follow the individual goals are specific requirements that promote attainment of the goal. The City of Prior Lake has maintained its natural drainage patterns throughout most of its development. The City s goal is to foster continued optimum use of that natural drainage system while enhancing the overall water quality entering the lakes. The intent is to prevent flooding while using identified best management practices to enhance surface water quality with minimal capital expenditures by the City. Upon approval of this LSWMP by the two watersheds with jurisdiction over the City, it is the City s intent to assume all permitting powers within it jurisdiction. Currently, the Scott County WMO does not issue permits, so no impact to this organization would City of Prior Lake ix
occur. The Prior Lake Spring Lake Watershed District does issue permits for any planned activity that disturbs more than 10,000 square feet of land area. This threshold rises to one acre if the activity is not near a lake, wetland, or the Prior Lake outlet channel. Since the watershed would still permit activities outside the City s jurisdiction its permit process would remain in place. Within its jurisdiction, the City will use the permit submittal requirements outlined in the watershed rules and updates. This will ensure consistency of approach for all projects. Once the LSWMP is approved, the city will enter into a memorandum of understanding, regarding the transfer of permit authority for the PLSWD to the city. The PLSLWD would continue in its role as a project review agency though it may defer to the City review process for projects that don t have a direct impact on Prior Lake or the Prior Lake outlet channel. The Prior Lake-Spring Lake Watershed District will also continue to have responsibility for water quality monitoring. The Prior Lake LSWMP envisions the City and its two watersheds as partners in implementing this plan. In the PLSLWD lands, the City envisions the watershed taking the lead on water quality and lake water quality issues. The City and watershed would be equally responsible for implementation of the volume management targets discussed in Section 5 of this Plan with the City taking the lead in the 2030 expansion areas and the watershed taking the lead in areas outside the 2030 boundary. The goal of this wetland inventory is the management of wetlands based on the functions they perform and to determine appropriate protection strategies for stormwater discharge to the wetlands if a land use change occurs that triggers a NPDES permit. Since smaller wetlands are not typically used as major components in a stormwater storage system, we focused our inventory on wetlands shown on the National Wetland Inventory (NWI) Map that were over 0.5 acres in size. The inventory and assessment of wetlands through the Wetland Management Plan (section 4) allows the city to set up priorities based upon wetland functions and values. This plan includes a wetland inventory and ranking system that will assist the city in establishing priorities and focusing available resources for wetland protection, enhancement and restoration. Because all wetlands have value, all are protected, to some degree, in this plan. The plan is designed to provide the following benefits: Provide wetland inventory, assessment, and management information: Aid in administration of the Wetland Conservation Act (WCA) by providing information regarding the wetlands functions: Enhance wildlife values of wetlands: Provide and enhance recreational values: Designate wetland restoration/enhancement opportunities: Protect wetlands and adjacent resources that provide valuable ecological support: City of Prior Lake x
Provide stormwater protection for wetlands. It should be noted that the wetland inventory has been created for planning purposes only. Regulation of activities potentially impacting individual wetlands will be based on a site-specific delineation of the wetland boundary as part of a proposed project. All of the inventoried wetlands within the study area were classified for Stormwater and Habitat Protection. Stormwater Protection standards are listed in Table 4.2 and 4.3 and Habitat Protection Recommendations are listed in Table 4.4. The Stormwater Protection Standards include Water Quality and Quantity Protection. The Habitat Protection Recommendations include Buffer Zones and No Grading Recommendations. Water quality plays a significant role in the overall quality of a wetland. When the quality of the incoming water declines, the wetland s plant community may change to fewer numbers of species and retain only those species that are tolerant of high nutrient and sediment loads. Once a wetland s plant community is changed, the wetland s character and ecosystem will change, often to a less valuable system in terms of biodiversity, habitat for wildlife, and aesthetic enjoyment. Pretreatment requirements have been developed to maintain the character of the wetland. BMPs can be used to accomplish the pretreatment requirements given in Table 4.2. In the recent past, surface water management plans have protected wetlands from nutrients but not water fluctuations or duration. In fact, it was common to use wetlands to reduce flooding potential through sizing storm sewer pipes to maximize bounce and detention time in wetlands. This plan addresses stormwater quantity impacts to wetlands by providing protection strategies to maintain the existing integrity of the wetland through special protection strategies for highly, moderately, and slightly susceptible rankings and are described in Table 4.3. Wetland restoration/enhancement sites were identified during the field inventory and will be further investigated at the time of development under the requirements spelled out in the PWDM and ordinance. The wetland restoration portion of the filled out MnRAM will be reviewed at the time of development to determine the potential for restoration of wetlands on the property. The potential for wetland restoration will be determined based on the ease with which the wetland could be restored, the number of landowners within the historic wetland basin, the size of the potential restoration area, the potential for establishing buffer areas or water quality ponding, and the extent and type of hydrologic alteration. Section 5 of the Plan serves two functions. The system assessment portion catalogues the various assessments of problems that the Plan must address whether they relate to water quality, wetland protection, flooding, volume management, or lakes management. The intent is to identify the source of problems and, more importantly, specific actions the City will take to address these problems either independently or in collaboration with City of Prior Lake xi
some other organization most commonly one of the watershed management organizations. The purpose of the system design portion of this section is to identify and quantify the infrastructure needed to allow continued development in Prior Lake while avoiding the negative impacts, such as flooding and water resource degradation, often associated with development. The system design portion of this section describes the 2030 growth area surface water management system and is intended to be used as a planning and analysis tool. This system is shown in maps 1 through 5. The discussion of the system revolves around answering the following questions: What are the general drainage patterns of the 2030 and existing system? What does the 2030 system entail in terms of storage, conveyance, volumes, and discharge rates? Where does the proposed system discharge and what constraints in the existing system limit discharge of the 2030 system? What is the impact of agricultural drainage, outside the 2030 growth area, on the proposed and existing urban system? How have proposed wetland bounce, and duration of HWL, been determined by management guidelines of the Wetland Management Plan, section 4 of the LSWMP? What opportunities exist for obtaining the retention storage identified by the PLSLWD both in the 2030 growth area and outside it? What is the impact of the City of Prior Lake s 2030 urban system on agricultural areas and other municipalities? Are there any existing ponds where calculated HWL is a concern? A number of water bodies within the existing City and its 2030 growth boundary are listed in the state impaired waters list. Known as the 303(d) list from the applicable section of the federal Clean Water Act, these waters are ones that do not currently meet their designated use due to the impact of a particular pollutant or stressor. If monitoring and assessment indicate that a water body is impaired by one or more pollutants, it is placed on the list. At some point a strategy would be developed that would lead to attainment of the applicable water quality standard. The process of developing this strategy is commonly known as the Total Maximum Daily Load (TMDL) process. When discussing nutrient impacts to lakes the nutrient most commonly identified is phosphorus. Through its own monitoring efforts and those of the Citizen Assisted Monitoring Program (CAMP) run by Metropolitan Council, the PLSLWD has been collecting data on nutrient loading into the impaired waters, and others, identified above. According to the PLSLWD 2003 Annual Report: All of the lakes in the District are either eutrophic or hypereutrophic except for Cates Lake and Lower Prior Lake, which are on the upper boundary of mesotrophy. Review of Table 4.5 and comparison with the TSI descriptions in Table 4.3 shows that both Cates Lake and Lower Prior Lake are very close to the boundary for a eutrophic lake, and this boundary is where problems really start to become evident. The western end of Lower Prior Lake is mesotrophic/eutrophic largely because of water flowing through this end from Upper Prior Lake to the outlet. The rest of City of Prior Lake xii
Lower Prior Lake has a limited watershed and is isolated from a majority of the inflowing water from Upper Prior Lake. The continued assessment of these lakes has led the PLSLWD to emphasize reduction in phosphorus loading to the lakes. This will also be the focus of a watershed-based TMDL, when developed, for the impaired waters listed in table 5.1. Since the mercury TMDL will be regional in nature, the City of Prior Lake and PLSLWD will focus their efforts on reducing nutrient loading. According to the PLSLWD: For noticeable improvements to occur in lake water quality, TSI values need to be reduced to 55 or less. On the reverse, if these lakes are allowed to decline further, algae blooms will become worse and fish kills are probable. In addition to collecting and reporting on the above data, the PLSLWD has created a model to quantify the internal and external phosphorus load for Spring and Upper Prior Lakes. This modeling effort is summarized in the 2003 Annual Report: In summary, sediment phosphorus release and recycling accounts for approximately 43 to 78% of the total phosphorus load for Spring Lake and 49% of the total phosphorus load for Upper Prior Lake. As a result, significant water quality improvements in each lake will require implementation of lake improvement options that would greatly minimize the potential for sediment phosphorus release. In addition, significant reductions in phosphorus from County Ditch 13 and Spring Lake should result in significant water quality improvements in Spring Lake and Upper Prior Lake, respectively. To a lesser degree, senescing macrophytes and bottom-feeding fish also affect the water quality of Spring and Upper Prior Lakes, since each of them contribute approximately 5 to 15% of the total phosphorus load to each lake. The Water Resources Management Plan for the PLSLWD, completed in 1999, identified several planning efforts, that would occur subsequent to the Plan, to address issues with the Prior Lake water levels and outlet operation. These included: Calibrating an hydrologic model for the watershed Designing improvements to the outlet channel for full-development conditions Addressing flood prone structures on Prior Lake Addressing increases in runoff volume as development occurs The PLSLWD report Prior Lake Outlet Channel and Lake Volume Management Study (May, 2003) addresses these issues in detail. The 100-year floodplain elevation for Prior Lake established by FEMA is 908.9 MSL. There are 79 homes around the lake with low openings lower than this floodplain elevation. Fifty-one of these have low openings below 907.6 and ten have low openings below or within one foot of the lakes 904.0 OHW. According to PLSLWD information, this 904.0 elevation has been exceeded a total of 259 days since 1983. Since development tends to improve drainage pathways and increase runoff volume, the impact of future development on Prior Lake could, without mitigation, increase the frequency of water levels above the 904.0 OHW. To assess the impact development might have on water levels in Prior Lake, the PLSLWD created a calibrated model of the watershed. The calibration of this model City of Prior Lake xiii
started with standard curve numbers for the subwatersheds tributary to the lake and, through the calibration process, modified these until modeled results matched monitored lake levels for the 1998 to 2001 period. The hydrologic modeling for the LSWMP is based upon this calibrated watershed model. The difference between the two, is that the LSWMP model looks at the conditions that will exist when build out occurs in the 2030 growth area. Additionally, the LSWMP model includes more detail on the storage and conveyance system necessary to serve the 2030 growth area. Subsection 5.4 provides specific issues in the surface water management system for Prior Lake s 2030 growth area. The study area has been broken into 21 major drainage districts, which are further divided into subdistricts. The nomenclature for the major drainage districts is based on the Prior Lake Spring Lake Watershed District (PLSLWD) modeling and major tributary water bodies. Total acreage for major drainage districts should roughly match the existing work completed by the Watershed District. Section 5.4 describes each drainage district in detail. Section 6, Implementation Plan, of the Prior Lake SWMP describes those activities and programs the City might develop toward improving its surface water management program. Capital outlay for the surface water system (pipes, channels, and ponds) shown on the system maps will be large. For this reason a financing mechanism, called an area charge, is developed in this section. Based on the Capital Improvement Plan and the developable acreage, an area charge is developed and application of this charge is discussed. The concept of an area charge to finance expansion of the trunk stormwater management system is not a new concept for the City. Since its report titled Trunk Storm Sewer Fee Determination Study (February, 2001) the City has quantified future trunk and ponding needs and developed an area charge based on actual costs of these needs spread across the potential developable acreage. With the analysis contained within the SWMP the City will update the fees for the 2030 growth area. Section 6 also includes: An overview of the City s NPDES permit A discussion of operation and maintenance procedures and strategies An outline of an education program Financial considerations for the storm water utility A section referencing applicable design standards for stormwater management A section on Watershed implementation priorities Implementation priorities for the City A discussion of the process for amending this plan and an annual report to council Appendix D summarizes the modeled system costs by element, by major watershed, and for the system as a whole. The potential system, as shown in system maps, carries an estimated cost of $14,858,788. and serves as a basis for development fees in the City. This cost includes indirect costs of 30% on trunk and pond construction and indirect costs of 10% on easement acquisition. City of Prior Lake xiv
Table 6.1 presents a financial model for the City of Prior Lake. The cost elements come directly from the 2030 stormwater system design as described in the system maps and the appendices to this report. The various trunk elements are organized by prospective year of implementation as well as whether they constitute a pond cost or trunk pipe cost. Total costs for the 2030 system are $10,836,957. It is important to note that the system analysis was complete to estimate costs, and does not represent final design. Sections 6.2 and 6.3 develop an area charge for the City of Prior Lake that can be applied to future development within the City. The area charge has been constructed methodically as follows: 1. Pond and trunk costs for near term development have been estimated. A stormwater CIP has been created as shown in appendix D and table 6.1. 2. Net assessable acreage has been determined. 3. The base area charge has been modified into a land use based area charge through the use of equivalent acres. A storm water system is a major investment for the City of Prior Lake both in terms of initial capital cost and in terms of ongoing maintenance costs. The capital improvement program outlines the costs for new trunk system construction which will be funded by area charges. System maintenance is funded by the city s storm water utility. The city s storm water system maintenance responsibilities include the following: Street sweeping Cleaning of sump manholes and catch basins Repair of catch basins and manholes Assessing pipe condition (typically by televising) Inspection of storm sewer inlet and outlet structures Pond mowing and other vegetation maintenance Excavation of accumulated sediments from ponds Maintenance or other structural BMPs owned by the City The city has maintained its pipe system for decades and staff has a strong grasp on the costs associated with this. As new development brings more ponds into the system, city staff will find that pond maintenance becomes an increasingly large portion of both staff time and maintenance budget. It is important to quantify the extent of this future commitment so that the funds necessary for pond maintenance activities can be collected via the city s storm water utility. The City of Prior Lake implemented a stormwater utility in 1993. The current quarterly residential charge is $6.00 per residential unit. Annual revenue from the stormwater utility has grown as shown in table 6.5. Generally, revenue has grown not because of increases in the charge (the charge has gone from $5.63 in 1997 to $6.00 in 2005, an increase of 6.6%) but due to development bringing in more properties over which to collect the charge. With this increased revenue, though, has come an increase in the City s maintenance responsibilities. City of Prior Lake xv
In the past the stormwater utility has funded a staff position, programs, and capital expenditures. The 2002 capital projects totaled $140,000 and included a dredging project, a lake bank stabilization project, and some storm drainage improvements. In order that storm water utility (SWU) funding keeps pace with increase in municipal maintenance responsibilities, the city should plan for the costs to conduct periodic pond maintenance. Limited data on maintenance activities has been developed by watershed management organizations. A review of this data suggests an annual maintenance budget of $1,250 per acre-foot of wet volume or $4,350 per acre of surface at NWL. Either parameter is relatively easy to track. This $1,250 per acre-foot maintenance item can be translated into a per household cost by virtue of the fact that one acre-foot is sufficient pond wet volume for 20 acres of residential development. Assuming 2.5 units per gross acre, then $1,250 per year is spread among 50 units - $25 per unit per year. The current residential rate is $24 per unit per year. The current charges provide approximately $300,000 per year in revenue of which only about $20,000 to $40,000 has been used for pond maintenance. As the city s maintenance responsibilities grow the storm water utility funding also needs to grow to keep pace. Prior Lake is a regulated MS4 under the Phase II NPDES Permit. There is a cost associated with preparing an NPDES permit and the associated Storm Water Pollution Prevention Plan (SWPPP). Some estimate cities the size of Prior Lake will spend $50,000 every five years for permit preparation. For Prior Lake it is reasonable to assume that $10 per household will be spent every five years adding $2 per year to the individual household s storm water utility bill. The NPDES permit and SWPPP commit the city to certain activities, including capital projects, for the purpose of improving the quality of the city s storm water discharge. The U.S. EPA has estimated that the financial commitments that city s will make may total $10 per household per year. Others place this figure at $20. Since many of the activities identified by the SWPPP may already be funded (like street sweeping and pond maintenance) the $20 figure is probably too high. For the purposes of planning increases in SWU collection the $10 per year figure should be used. Table 6.5 summarizes the additional storm water utility charges identified above. The City of Prior Lake s implementation priorities include building the stormwater management system described in this report. Other implementations priorities relate to downtown redevelopment, retention storage, and adequate funding. Originally, City Prior Lake and Prior Lake Spring Lake Watershed District staff discussed the possibility of creating an overall stormwater management plan for the downtown area that, once approved by the Watershed, would allow the City sole permit authority for construction and reconstruction projects in the downtown area. In 2003, as the City moved forward designing the rainwater gardens for Erie Street and City Hall and the street reconstruction project for downtown it became apparent that how downtown redevelops, and what sort of water quality and quantity retrofits can be implemented, depends on factors that cannot be adequately quantified at this time. Consequently, at City of Prior Lake xvi
that time the City opted not to submit such a plan but rather would have the Watershed permit the specific projects. In 2007 the City will undertake a downtown stormwater management study in cooperation with the district. Provisions for downtown permitting authority will be spelled out in a MOA with the District. The results of the 1007 study will begin to be implemented in 2011 or when the downtown area reconstruction begins. In the interim, the City will continue to look for opportunities to retrofit small site BMPs, water quality improvements, and rate control improvements as warranted by downtown redevelopment activities. Other implementation priorities for the City as it adopts this Plan and begins the implementation phase of the Plan include: 1. Assisting the PLSLWD in implementing its retention storage program. Specific areas with high potential for City implementation are indicated on the system maps and within the body of this Plan. 2. Increasing Storm Water Utility Funding so that the City can meet its current and future obligations toward pond maintenance, NPDES compliance, and mitigation that may come out of the City s non-degradation analysis. 3. Application of the revised area charge outlined in this report and update of the area charge based on increases in land value and construction costs. 4. Implementation of the rate control targets as outlined in the appendices and stormwater modeling that supports this plan. 5. Application of the wetland susceptibility criteria in determining how wetlands are used for flood storage, retention, and rate control. 6. Working with the PLSLWD regarding the feasibility of augmenting storage in Buck Lake. 7. Working with the City of Shakopee toward redefining rate control objectives from their Sand Creek drainage which will ultimately enter the City of Prior Lake system through its Louisville Swamp system. The Prior Lake SWMP is intended to extend through the year 2015. For the plan to remain dynamic, an avenue must be available to implement new information, ideas, methods, standards, management practices and any other changes that may affect the intent and/or results of the SWMP. A brief annual report will be made by City staff summarizing development changes, capital improvements, and other water management-related issues that have occurred over the past year. The review will also include an update on available funding sources for water resource issues. Grant programs are especially important to review since they may change annually. These changes do not necessarily require individual amendments. The report can, however, be considered when the plan is brought up to date. The annual report should be completed by July 1 st to allow implementation items to be considered in the normal budget process. City of Prior Lake xvii
The following recommendations are presented for the City Council's consideration based upon the data compiled in this report: 1. The Surface Water Management Plan as presented herein be adopted by the City of Prior Lake. 2. Standard review procedures be established to ensure all new development or redevelopment within the City is in compliance with the grading and stormwater management controls determined by this Plan. 3. Detailed hydrologic analyses be required or all development and redevelopment activities. 4. Final high water levels governing building elevations adjacent to ponding areas and floodplains be established as development occurs or when drainage facilities are constructed. 5. Overflow routes be established and maintained to provide relief during extreme storm conditions, which exceed design conditions. 6. A surface water system maintenance program be established to ensure the successful operation of the system. 7. The erosion and sedimentation control criteria for new developments be enforced. 8. An education program for City residents, staff, and development community be implemented. 9. Amendments to the plan be adopted and implemented as warranted by future standards or regulations. 10. That the plan be updated in 2010 or earlier if needed. 11. Promote the use of small-site/distributed BMPs to help achieve water quality and volume control goals. 12. Pursue partnerships with watershed management organizations and other agencies to incorporate volume control BMPs into re-development projects, including City projects. 13. Ordinances be revised to be consistent with rules detailed in the PWDM regarding water resource management. City of Prior Lake xviii
1. INTRODUCTION 1.1 Background This report provides the City of Prior Lake with a (LSWMP) that will serve as a guide to managing the surface water system throughout the City. The LSWMP is intended to complement the City s 2030 land use plan as formalized in the Comprehensive Plan 2030. The LSWMP will carry the City through the end of 2015. Periodic amendment to the Plan will occur in the intervening 10 years so that the Plan remains current to watershed plan amendments and revisions and current to the state of the art in surface water management. The City of Prior Lake is located in north central Scott County at the nexus of major transportation corridors including Trunk Highway 13, Scott County Road 21 and Scott County Road 42. According to some estimates, Scott County is the 15 th fastest growing county in the nation, and Prior Lake has seen a good portion of this growth. The Village of Prior Lake was incorporated in 1891. In the period from the 1870 s to the 1920 s little growth occurred in the City. During this period, much of the activity in the Village related to Prior Lake and its role as one of the preeminent recreation destinations of that era. The lake remains a recreational focus and forms one part of the City s threefold recreational system: 1. Lakes and natural areas 2. Parks, trails and active recreational facilities 3. Venues such as Mystic Lake Casino and the City s golf clubs It has been since the 1960 s, and the City s emergence as a suburban community, that the City s growth has escalated with the greatest growth occurring in the last 20 years. Table 1.1 provides City populations and population projections from 1980 through 2020. As the city continues to grow, the importance of adequate surface water management controls also grows. The intent of the Prior Lake LSWMP is to detail what these controls are and make the connection between these controls and the overall city goal of preserving and enhancing its natural resources and protecting its residents from flooding. City of Prior Lake 1-1
Table 1.1 Population and Households Year Population Number of Households 1980 7,284 2,313 1990 11,482 3,901 1995 13,427 4,630 2000 16,034 6,167 2010 21,000 8,077 2020 28,445 10,971 Lakes define the City since the core of the community developed between Upper and Lower Prior Lake. The City s uniqueness is tied to the lake. Consequently, effective surface water management cuts to the core of the City s vision for the future. 1.2 Purpose and Scope The Prior Lake LSWMP will serve as a comprehensive planning document to guide the City in conserving, protecting, and managing its surface water resources. The LSWMP meets requirements as established in Minnesota Rules 8410. In addition, the participation of other organizations, particularly Scott County WMO and Prior Lake Spring Lake Watershed District, ensures the City s compliance with local and regional expectations. According to the 1999 PLSLWD Plan local plans must do the following: Describe existing and proposed environment and land use Provide a narrative addressing stormwater infrastructure philosophy, which details regulatory authority, and implementation and financial responsibilities. Define areas and elevations of stormwater storage adequate to meet performance standards established in the watershed plan Identify quality and quantity protection methods which meet standards Identify regulated areas and potential easements or land acquisition areas Outline a procedure for submitting annual reports to agencies which document Wetland Conservation Act and monitoring program data consistent with state compatibility guidelines Set forth an implementation program, including a description of official controls, inspection and maintenance, and capital improvement plan Describe official controls and the responsible unit of government in the following areas: wetlands, erosion control, shoreland, floodplain, grading, and drainage The City will submit its LSWMP to Metropolitan Council, Scott County WMO, and PLSLWD for review. These entities have 60 days for their review after written receipt of the City Plan. In a four-part process, the Prior Lake LSWMP does the following: Collects and compiles the efforts of agencies and organizations including the City, its departments and residents. This includes past reports and studies, management City of Prior Lake 1-2
plans, monitoring studies, as well as completed and proposed improvement projects. Reviews the current state of the City s surface water resources in the context of goals and policies, ordinances, operations and maintenance, flood mitigation, and achievement of targeted water quality levels in its surface water bodies. Establishes reasonable, achievable and affordable goals, and supports them by a strong regulatory and management culture. Develops an implementation plan that includes projects and processes that derive from a thorough assessment of current City problem areas and current City surface water regulations and controls. Provides a blueprint for construction of new surface water systems as the City expands into its 2030 growth area. Using advanced surface water modeling software, a system of pond, wetlands and pipes is developed and costs applied to these future systems. The costs give the City a framework for understanding the impact development will have on City finances and applying these costs equitably to development. In order to arrive at a LSWMP that adequately addresses surface water related issues, the emphasis has been to work with others to identify important issues through review and meetings. City staff has participated in collecting data, providing feedback, and contributing knowledge of local systems to aid in developing a strategy that encompasses water quality and quantity issues. The City of Prior Lake is the organizer of the final document though contributions from the watersheds have been substantial. Two other activities complement the LSWMP. The Wetland Management Plan (WMP) provides an assessment and management plan for numerous wetlands within the 2030 growth area. The WMP is based upon standard assessment methodology and is utilized, in conjunction with the LSWMP hydrologic modeling, to determine future use of wetland basins for storage, retention, and infiltration. The WMP constitutes section 4 of this report. The second activity is the Upland Management Plan. Like its wetland counterpart, this Plan provides an assessment and management plan for resources this time upland resources. The City will utilize this information in open space and park planning. Based on the guidance provided by the Prior Lake city council and staff, this report addresses the city s current surface water management needs and provides a framework for successful implementation of a comprehensive storm water management program. A specific outline of the steps involved in the preparation of the SWMP is presented below: 1. System Inventory and Mapping Analyze drainage patterns and develop a trunk storm water system map for the 2030 drainage system. 2. Goals, Policies and Guidelines Develop goals and policies that guide the city s surface water management philosophy. Augment design guidelines for development and redevelopment. This gives the City guidance for facilities design and standards for reviewing development plans. Included in this process is determining all regulatory agencies involved in the storm water City of Prior Lake 1-3
management of the City and working with City staff to develop feasible goals, policies and guidelines. 3. System Analysis and Design Analyze the storm water system and develop a recommended system. A system model was created that extended beyond the city s current boundary into future development areas. This step also includes specific recommendations for system upgrades and improvements due to erosion and flooding issues. 4. Cost Estimates and Capital Improvement Program For the recommended system, develop itemized cost estimates of facilities. These are planning-level estimates, suitable for budgeting and decision making. Feasibility studies will be needed for more detailed costs. In addition, a 5-10 year CIP has been developed to coordinate system construction with growth. 5. Storm water Ordinances Recommend ordinances or revision to existing ordinances. 6. Storm water System Management Provide recommendations on operating and maintaining the storm water system as well as best management practices (BMPs) for water quality and erosion control. Information regarding compliance with NPDES Phase II Storm water Permits is also included. Prior Lake presents something of a contrast. In older parts of town, city staff must meet the challenges of maintaining an older storm drainage system. In other areas, new development is adding new infrastructure to the storm water system and thus increasing the city s maintenance responsibilities. The SWMP is primarily aimed at this new development and the goals, policies, guidelines, controls, and preliminary system design reflect that emphasis. 1.3 Organization This report is a culmination of the activities described above and is organized as follows: Section 2, Land and Water Resources Inventory, describes the physical environment including watersheds and drainage patterns, dominant land uses, and significant water bodies within the City. Section 3 - Goals, Policies and Guidelines - lists the City s goals and policies along with public agency requirements affecting surface water management in the City. Section 4, Wetland Management Plan, presents the results of an assessment of the City s larger wetlands within the 2030 growth areas. The Wetland Plan identifies specific strategies for mitigating wetland impacts often associated with development. Section 5, System Analysis and Design, presents an overview of all the major watersheds in the City. This section describes in detail the affect rural drainage has on municipal systems both now and in the future. Section 4 also provides detail on the proposed storm water management system within the four focus areas. The focus areas are soon to develop portions of the larger study area where specific ponds and trunk pipes are sized and shown, where trunk alignments are City of Prior Lake 1-4